Coated surfaces capable of decomposing oils

ABSTRACT

The present invention provides a coated metal substrate useful for decomposing cooking residues, e.g. oils through a gasification cracking reaction by providing a specific catalytic surface. The catalyst is at least one of Group 1A or 2A oxides or compounds of the formula (MA) x  (MB) y  (O) z  where 
     MA: element of Group 1A or 2A 
     MB: element of Group 3A or 4A 
     O: Oxygen 
     x, y, z: integer 
     said catalyst being the sole effective catalyst for decomposing oils and being present in an amount sufficient to effect such decomposition and an inorganic binder, a hardening agent, and a pigment.

This application is a divisional of application Ser. No. 480,380, filedApr. 5, 1983, U.S. Pat. No. 4,471,027 which is continuation of Ser. No.263,177 filed May 13, 1981, now abandoned, which is a continuation ofSer. No. 85,059, filed Oct. 12, 1979, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to coated surfaces designed tocatalytically remove food and other cooking residues through applicationto the oven of the cooking device or the like. To effectively performcatalytic action on the coated surfaces using an oxidation catalyst,sufficient contact between the catalyst and oxygen (air) is required.However, it is difficult to simultaneously provide, for the surfaces,thereof abrasion resistance as well as surface characteristics such ashardness or the like, and catalytic effect. The amount of the residuesscattered on the inner walls of the cooking device oven is beyond thecatalytic capacity during the real cooking operation. The surfaces arecovered with the residues in a short time, thus immediatelydeteriorating the catalytic effect.

The temperatures of the oven inner faces of the cooking devices areapproximately 200° to 300° C. The reaction between the oxidationcatalyst and organic residues is likely to causeoxidation.dehydrogenation reaction, instead of complete oxidationreaction, to make the organic matter carbonaceous, which is difficult toclean off. The present inventors have considered that the conventionalcleaning, coating technique primarily using the oxidation catalyst isnot practical. Since the cooking residues such as fatty acids or thelike ordinarily have boiling points of approximately 200° to 300° C.,the skilful use of evaporation and furthermore the gasification throughdecomposition of these compounds into compounds each having a boilingpoint less than 200° to 300° C. will serve to effectively clean thecooking residues. Accordingly, the present invention seeks to providecatalysts for the removal of such residues along the lines discussedabove.

SUMMARY OF THE INVENTION

A major point of the present invention relates to a catalyst foreffectively gasifying the fatty acid or the like at temperatures of 200°to 300° C. Various conventionally-known surface treating techniques canbe applied as a support for effectively dispersing and supporting thecatalyst on the surfaces. As preferred examples in practical applicationan, inorganic coating with alkali metal silicate being used as a binder,inorganic coating with metallic phosphate being used as a binder, or thelike are provided. As a gasified decomposition catalyst for the fattyacid,

(A) oxides of 1A group or 2A group in a periodic law table

(B) compounds represented in an equation (MA)_(x) (MB)_(y) (O)_(z)

wherein

MA: element of Group 1A or Group 2A

MB: element of Group 3A or Group 4A

O: oxygen

x, y, z: integer

a compound of the kind or more selected from the (A) and (B) groups or acompound of one kind oxide or more of an element selected from a groupof Ti, Fe, Ni, Co, Cr, Ag is effective.

When the catalyst exhibits effective results on the support of a matrixcoating material, mixing conditions between the catalyst and theinorganic coating of the support is important. If the catalyst and theinorganic coating of the support are completely mixed, the catalyst isenveloped in a film and cannot provide an effective result. Namely, themixing conditions for the catalyst and the inorganic coating of thesupport for the catalytic action should be incomplete where the catalystdoes not enter the coating. Also, since the decomposition reaction withthe catalyst does not require oxygen, the porosity of the surface is notrequired to be higher than in the case of the oxidation catalyst.Accordingly, coated surfaces which are extremely rigid and durable canbe formed.

DETAILED DESCRIPTION OF THE INVENTION

U.S. Pat. No. 3,460,523 (Stiles) has proposed coating surfaces tocatalytically clean the food residues from ovens. Stiles hassuccessively proposed a method of dispersing the oxidation catalyst intoglass frit, instead of a water glass group bonding agent, and the othermethods, which are in practical use. At the present time, Stiles' formermethod is inferior in abrasion resistance or corrosion resistance whenevaluation has been made in terms of surface film, and is inferior inthat the accumulation capacity of the residues on the coated surfaces issmaller due to relatively thin film. Continuous self-cleaning of anenamel type becomes more prevalent. The latter method is mainly used.Representative of the latter methods is a proposal by Lee et al. U.S.Pat. Nos. 3,547,098, 3,576,667 and 3,598,650. Not only food, but alsoseasonings or the other materials are responsible for dirty ovens.Particularly, a material which causes not only soiling but also variousother unpleasant conditions, is fat. The conventional method uses anoxidation catalyst which has superior catalyst activity with respect tothe oxidation reaction, to catalytically oxidize cooking-residue organiccompounds including the fat for cleaning applications.

However, the oxidation catalyst is delicate. The cooking residue organiccompound is removed when the compound has been heated in air for, atleast, approximately one hour at 600° C. When the organic compound issubjected to such conditions, evaporation and thermal cracking reactionsimultaneously occur and furthermore oxidation also occurs. In the formof the oxidation reaction, oxygen bonds with the organic compound and inanother form, dehydrogenation occurs. Through the visual evaluation, theorganic compound is gradually carbonized and finally the carbon isgradually oxidized.

The oxidation catalyst can activate the oxidation reaction toeffectively perform the oxidation reaction at lower temperatures.However, at a temperature of 600° C., the same process as where theorganic compound has been removed only by heat connot be performed atlower temperatures. In a case where organic compound residues similar tocarbon are fired at 600° C. and cleaned, the carbon is oxidized anddisappears in a form close to combustion. At temperatures of 200° to300° C. where the oven condition of the cooking device is provided asdescribed as hereinabove, it is not easy to perform the oxidationreaction, which burns the carbon, using the oxidation catalyst.

Also, it is inferred that the contribution of the oxidation catalystprovides complete oxidation reaction, namely, the organic compound canbe completely oxidized into water and carbonic acid gas. However, thisinference is unreasonable.

The present invention is completely different from the conventional onewhere the cleaning is performed using the oxidation catalyst. Namely,the residues such as oils or the like are inconvenient on the surfaces,since they exist in liquid state or solid state. The residues on thesurfaces are removed when they are changed into gaseous state to bereleased from the surface. When the oils in solid state are caused tochange into gas condition at temperatures of 200° to 300° C., thecatalyst used is not required to have stronger activity. The superioreffect can be provided with a catalyst which activates the gasificationcracking reaction. On the basis of this concept, a catalyst such asdescribed hereinabove has been discovered.

In the conventional technique, manganese, iron, cobalt, zirconium,chromium, copper, which are metallic oxide having strong oxidationcapability, or rare earth metallic oxide or the like are used asoxidation catalysts. Accordingly, the conventional disadvantages arethat the color of the coated surfaces is restricted to black as a maincolor, the surface hardness becomes lower and abrasion resistance isextremely deteriorated due to the requirement of sufficient exposure ofthese catalysts to the film surface to effectively exhibit the catalyticcapability, or the film becomes fragile due to requirement of higherporosity which is indispensable in order to sufficiently diffuse theoxygen to the oxidation catalyst.

According to the present invention, it is required to control theoxidation reaction as much as possible in the cleaning reaction of theoils to perform the gasification cracking reaction. As for the reaction,the supply of the oxygen is not required. Since the reaction isperformed between the oil of liquid phase and the catalyst on thesurfaces, porosity at a level where the oxidation catalyst is used isnot required. Thus, the film to which the catalyst is not added is notrequired to give up its own original nature due to porosity.

Since the gasification cracking catalysts in a system discovered by thepresent invention are mostly white, it is possible to color the surfacesto an optional color through combination with a proper pigment.

The present inventors had doubts that salad oil or the like could beoxidized into water and carbonic acid gas upon contacting of the saladoil or the like in liquid state with the conventional oxidationcatalytic surface. On this type of conventional enamel coating weanalyzed gas produced through thermal cracking in air under contact withthe salad oil (soy bean oil), using a gas chromatograph. No carbondioxide gas was detected through variation in conditions (after beingleft for one hour with temperature up to 350° C.). This fact indicatesthat the contribution of these oxidation catalysts is quite differentfrom the complete oxidation.

As a process as for causing the oil heated to become tarry in air, thereare considered a process where it is partially oxidized and ispolymerized through an intermediate such as peroxide or the like or aprocess where it is dehydrogenized and then is depolymerized through anolefin stage or the like. According to the inferences of the inventors,when a strong oxidation catalyst has been introduced into such reactionsystem, strong dehydrogenation occurs to promote tar or hardening. Fromsuch a point of view, the use of oxidation catalyst is not indispensableas a method of cleaning the cooking-residue organic compound. Thecleaning operation may be made in quite a different direction. Aneffective material was investigated from a view point of the thermalcracking capability of the salad oil (soy bean oil). Methane, ethylene,ethane, carbon monoxide, formaldehyde, etc. were produced as gaseousdecomposition products. It was determined by analysis that crackedhydrocarbon, which was different from composition contained in the saladoil itself, in addition to the above-described elements was produced inthe thermal cracking operation under air co-existence of the salad oil.

In the test conditions, 1.0 μl of salad oil was mixed, using amicrosyringe, with about 2 mg of compound particles. They weredecomposed, for ten minutes at 300° C., inside a closed glass container.Thereafter, the produced gas was introduced into a gas chromatograph foranalysis.

As the analysis conditions, N₂ carrier, (60 ml/min) a FID detector (H₂flow rate: 60 ml/min, air flow rate: 0.5 l/min) were used. As the columnconditions, silicone GE SE-30.5% liquid phase (Shimalite W. carrier) of3 mmφ×3 m was used. It was retained, for five minutes, at 150° C. andthereafter the temperature-rising analysis was performed till 250° C. atthe temperature-rising speed of 5° C./min to detect thecracking-produced gas.

Under the above conditions, a cracked decomposition product was detectedat retention times such as 100, 106, 139 and 173 minutes although theidentification was not reached (two former compounds are considered asmethane and ethylene).

The integration results (numeral values integrated by the use of digitalintegrators, i.e., the total sum of coefficient values of the fourcracked products) of detection peak area about the representativematerial are shown in Table 1.

Metallic oxides of Groups 1 to 3 of the periodic table, and particularlyoxides of Group 1A Group 2A alkali metal and alkali rare earth metalsare provided as metallic oxides or compounds showing superior catalyticactivity for the decomposition in air of the salad oil (soy bean oil),in accordance with Table 1.

                  TABLE 1                                                         ______________________________________                                        Comparison between salad oil decomposing                                      capabilities of various compounds                                                   Metal Group                                                                              Coef.           Group of                                                                              Coef-                                      of Periodic                                                                              ficient         Periodic                                                                              ficient                              Oxides                                                                              law table  values  Compounds                                                                             law table                                                                             values                               ______________________________________                                        blank --         6871    K.sub.2 CO.sub.3                                                                      1A      21246                                                                 4A                                           CaO   2A         14332   CaSiO.sub.3                                                                           2A      58626                                                                 4A                                           Na.sub.2 O                                                                          1A         18756   CaAl.sub.2 O.sub.4                                                                    2A      58804                                                                 3A                                           K.sub.2 O                                                                           1A         20211   MgSiO.sub.3                                                                           2A      56273                                                                 4B                                           MgO   2A         40555   KAlO.sub.2                                                                            1A       8285                                                                 3A                                           CuO   1B          950    BaSiO.sub.3                                                                           2A      48018                                                                 4A                                           TiO.sub.2                                                                           4B         7800    BaTiO.sub.3                                                                           2A      10362                                                                 4B                                           NiO   8          9559    Li.sub.2 SiO.sub.3                                                                    1A      12538                                                                 4A                                           MoO.sub.3                                                                           6B         4058    Na.sub.2 CO.sub.3                                                                     1A      13855                                                                 4B                                           Fe.sub.2 O.sub.3                                                                    8          21697   CaCO.sub.3                                                                            2A       6678                                                                 4A                                           Co.sub.2 O.sub.3                                                                    8          7416    Ni--MgO 8        9471                                                                 2A                                           MnO.sub.2                                                                           7B         100 or  Ca(OH).sub.2                                                                          2A      19261                                                 less                                                         Cu.sub.2 O                                                                          1B         4600                                                         ZnO   2B         5563                                                         Cr.sub.2 O.sub.3                                                                    6B         8367                                                         Al.sub.2 O.sub.3                                                                    3A         5350                                                         SiO.sub.2                                                                           4A         5424                                                         Ag.sub.2 O                                                                          1B         13306                                                        SnO.sub.2                                                                           4A         6608                                                         ______________________________________                                    

Referring to Table 1, the superior compounds are compounds representedby the formula (MA)_(x) (MB)_(y) (O)_(z). It can be understood that MAis preferably an element from Groups 1A or 2A, and MB is preferably anelement from Groups 3B or 4B.

Particularly, when MA is composed of Na, K, Ca, Mg and MB is composed ofC, Si, Al, it is found out that the best result can be obtained. MnO₂ orCuO, which has been well-known as a so-called oxidation catalyst,interferes with the decomposition in this form.

In the case of these strong oxidation catalysts, it is also consideredthat dehydrogenation occurs and the decomposition occurs in a differentform.

However, the use of MnO₂, CuO or the like in their active condition isavoided for our objectives.

Also, metallic oxides such as Ti, Fe, Ni, Co, Cr, Ag, among metallicoxides exhibit activity in the gasification decomposition. Particularly,iron oxide exhibits superior activity.

Since the effects of these catalysts remain unchanged even in an N₂atmosphere, it is considered that the thermal cracking of the fatty acidis activated in a form irrelevant to oxygen.

The catalyst of the present invention is required to be added so thatthe catalyst may exist, in proper dispersion, in a surface coatingmaterial having proper thermal resistance. The surface coating materialbase is applicable to the well-known various coatings. It may be waterglass group compound proposed by Stiles or glass frit proposed by Lee etal., discussed above. In addition, it is applicable to various thermalresistance paints or enamel.

As a surface coating material base which is industrially lower in costand can provide a superior coating, a coating system using an inorganicbinder is provided. As a representative coating systems alkali silicateor metallic phosphate is used as a binder.

It is known from Stiles, supra, that the alkali silicate is used asbonding agent for various objects. Stiles recognizes that it has beenknown to the persons skilled in the art to use metallic oxide grouppigments in a substantial amount for coloring in the inorganic paint,with the alkali silicate as a major composition.

In most cases, these pigments are composed of metallic oxides orcompounds which satisfy such oxidation catalyst conditions as stated byStiles, such as zirconium, titanium, vanadium, chromium, manganese,iron, cobalt, nickel, tungsten, molybdenum, copper, zinc and rare earthoxide, and the precious metals, i.e., palladium, rhodium, ruthenium,osmium, iridium, and platinum, and their mixtures.

Although the conventional silicate group inorganic paints containsmetallic oxide of 10% or more and silicate compound of 5% or more,effects reported by Stiles are not provided at all, because the metallicoxide used in the conventional paint is in the form of a compoundsuitable as a pigment, but not as a highly active catalyst which is highin reactivity; and the metallic oxide of the pigment is mostly wrappedin a film of alkali silicate which is free fro catalyst activity asalready recognized in the catalyst circle before Stiles' proposal, sothat Stiles' catalytic effect cannot be exhibited.

Most pigments of the inorganic paint are considered to have been wrappedin a film of the alkali silicate, because the the coated surfaces of thepaint are extremely hard and, similar to hardness of the single film ofthe alkali silicate; or the surfaces are flat, being free fromunevenness as directly observed by a scanning type of electronicmicroscope.

According to Stiles an adjusting method was adopted which involvesstirring and mixing the silicate group bonding agent and the catalyst,whereas in the single inorganic paint prior to Stiles' invention, thesilicate group bonding agent and the pigment were mixed for at least 48hours, using a powerful mixing means such as ball mill or the like. Thecoated surfaces provided through application of the paint, adjusted inthe method of the latter, on the metallic surfaces become almost flat,being almost free from porosity with the pigment being almost wrapped inthe film of the silicate.

According to the evaluation of the present inventors, thecharacteristics of Stiles' invention, which are different from theconventional inorganic paint, are that the mixing between the metallicoxide and the silicate group bonding agent is caused to be sufficientlyperformed for complete resolution or the mixing operation therebetweenis insufficiently performed so that they may hardly be resolved. Sinceit is difficult to conclusively evaluate the conditions of preparationonly from the finished coating film, the evaluation depends upon theporosity only to clearly distinguish the difference. In this sense, theporosity is very important to Stiles' invention. As describedhereinabove, Stiles' method is inferior as a surface coating due to thisporosity and is inferior to the enamel type.

The effect of the present invention will be described hereinafter withreference to the following embodiments.

EMBODIMENT 1

As a silicic acid compound on the market, lithium silicate sol ("NissanChemistry LSS-45") was used. As a compound which was effective in Table1, CaO, MgO, CaSiO₃, CaAl₂ O₄ were used and were mixed so that theproportion of each catalyst component becomes 20%. It was applied, witha brush, on a test piece (SPC-1) of 10 cm² and was baked at 200° C. for30 minutes. Thereafter, at 550° F. (approximately 288° C.), the saladoil of approximately 3 μl was repeatedly dropped, as a spot, forevaluation of cleaning capability. In either case, complete cleaning wasconfirmed, without any trace, after 20 minutes. As they were white, theextent to which they were cleaned was easier to be observed. The oil,when colored grey, was thereafter cleaned so that traces thereofgradually disappeared.

EMBODIMENT 2

Since the surface physical property of Embodiment 1 was not alwayssuperior in adherence property, etc., sodium silicate group paint"Ceramitite" of Shikoku Kaken Industry Ltd., Japan was used as a heatresisting paint, and was satisfactory in physical properties. Varioustypes of paints were available. Fundamentally, the paints arerespectively composed of a water glass bonding agent, main agent withpigment and filling agent as major components, and hardening agent withaluminum phosphate, zinc oxide, etc. as major components. Paint, whichhad no pigment was prepared using "Ceramitite:King". CaSiO₃ : 6 parts byweight and CaAl₂ O₃ : 24 parts by weight were added as catalyst to thepaint 100 parts by weight. They were mixed by ball mill to preparepaint. The milling time in this case is preferably within one hour. Thispaint was applied on an aluminum treated steel plate of 10 cm in square(plate thickness 4 mm) and was baked for one hour at 300° C. The sametest as that of Embodiment 1 was carried out on the test piece. Thecoated film was 100μ in thickness and was flat, being almost free fromporosity. In a cleaning evaluating test, oil stains were cleaned aftertwenty minutes. The physical property of the coated film was extremelysuperior and was the same in adherence, abrasion resistance, waterresistance, humidity resistance, vapor resistance, heat resistance, heatimpact resistance or the like as that of the conventional enamelproduct. When the "Ceramitite:King" (black type: containing 30%composite metal oxide group pigment) was applied, the oil remainedtarry, thus resulting in no cleaning effect.

EMBODIMENT 3

In this embodiment, as a pigment-containing paint, a test was made withthe same "Ceramitite" (white type: containing approximately 10% TiO₂ aspigment) (however, in this case, TiO₂ is milled, as paint, with ballmill for fourty-eight hours or more, and thus it is considered to becompletely wrapped in silicate film) as base. Since the paint is white,coloring(blackening) due to tar formation in the salad oil or the likeis evaluated. A test piece (40 mm×80 mm×0.6 mmt) was disposed on a hotplate, which was set to approximately 250° C. Approximately 30 mg ofsalad oil was added as a spot so that the salad oil might bedistributed, in approximately 1 mg/cm², on the test piece. The cleaningproperties were observed for various coatings.

When the catalytic system of the present invention is used, the porosityis not always required to be high as described already. However, someporosity is required to effectively promote contact with the thecatalytic surface. In this sense, a higher porosity is preferred withina range such that the physical property of the film may not bedeteriorated. It is considered that approximately 10% porosity will do.

Comparative data are shown, in Table 2, regarding the cleaningcapability of the salad oil at 250° C. in various coating systems.Cleaning factor indicates corrected blank (where only the salad oil isleft on iron plate not treated) reduction, and is calculated from weightchange before and after thirty minutes' test. The film is thermallytreated in advance so that a constant amount may be provided at the sametemperature.

2% potassium carbonate by weight, 1% calcium oxide by weight, 2% calciumsilicate by weight and 2% alumina cement by weight were added to thelast system described in Table 2, i.e., the paint "Ceramitite". Inaddition, a system where 6 ml/100 g paint glycerin was added was best.Traces completely disappeared in a period of thirty minutes' testing.Since palmitic acid, with a boiling point of 271° C., or the like arecontained, considering the composition of the salad oil, 100% cleaningaction is considered due to a fact that it was thermally cracked even at250° C. due to the catalytic effect and was evaporated as a low boilingpoint material.

                  TABLE 2                                                         ______________________________________                                        Comparison between salad oil cleaning                                         capabilities of various catalytic coatings                                                          Glycerin                                                                      addition  Cleaning                                                            (ml/100 g factor                                        Coating system        paint)    (%)                                           ______________________________________                                        "Ceramitite" + 5 wt % K.sub.2 CO.sub.3                                                              none      68                                            "Ceramitite" + 5 wt % K.sub.2 CO.sub.3                                                               6        75                                            "Ceramitite" + 5 wt % CaSiO.sub.3                                                                   10        83                                            "Ceramitite" + 5 wt % alumina cement                                                                10        85                                            "Ceramitite" + 5 wt % CaO                                                                           10        80                                            "Ceramitite" + 2 wt % CaO                                                                           10        83                                            + 2 wt % K.sub.2 CO.sub.3                                                     "Ceramitite" + 2 wt % CaSiO.sub.3                                                                   10        86                                            + 2 wt % CaO                                                                  "Ceramitite" + 2 wt % CaSiO.sub.3                                                                   10        90                                            + 2 wt % alumina cement                                                       "Ceramitite" + 2 wt % K.sub.2 CO.sub.3                                                              10        93                                            + 2 wt % CaSiO.sub.3                                                          "Ceramitite" + 2 wt % K.sub.2 CO.sub.3 +                                                              6       97                                            1 wt % CaO+ 2 wt % alumina cement                                             "Ceramitite" + 2 wt % K.sub.2 CO.sub.3 +                                                             6        100                                           1 wt % CaO + 2 wt % CaSiO.sub.3 +                                             2 wt % alumina cement                                                         ______________________________________                                    

In addition, the same test was carried out using lard, with the sameresults. However, the lard was larger in its cleaning factor value.

Calcium oxide or the like acts as a hardening accelerator concerning thehot life which becomes a problem in practical application in terms ofpainting. 10% addition of the calcium oxide produces obstacles. However,the total addition amount of these catalysts, if 8% or less, hardlycauses problems in practical application.

Concerning alkali metallic compound such as potassium carbonate or thelike, where a material to be coated is aluminum or the like, the filmalkalinity is enhanced and blistering may be caused by hydrogen producedthrough reaction during the baking operation or film whiteningphenomenon (excessive alkali reacts with vapor, carbonic acid gas or thelike in atmosphere to produce white compound) may be caused.Accordingly, no addition is sometimes better.

The coated face itself thus formed is almost the same, in physicalproperty, as the conventional enamel products.

EMBODIMEMT 4

A catalyst was added to "Sumiceram P Type", phosphate group inorganicpaint, as base of Sumitomo Chemistry Company. Then, a test was made. Thepaint uses aluminum phosphate as a binder. The aluminum phosphate isacid. When the gasification cracking catalyst is added to the aluminumphosphate, the binder may react with a catalyst, if the catalyst is abasic compound, to gel the paint.

At first, only the paint was applied (film of approximately 100μ thickafter drying) on aluminum treated steel plate (10 cm in square, 0.4 mmtin thickness). Tha baking operation was performed for ten minutes at200° C. after ten minutes' drying operation at 100° C. Finally, thebaking operation was performed for thirty minutes at 300° C. This filmwas superior in adhesion, water resistance, abrasion resistance, thermalimpact resistance, heat resistance, steam resistance, chemicalresistance, stain resistance, corrosion resistance and the like, but wasnot capable of cleaning the oils.

We evaluated the oil cleaning capability by the following tests. Namely,a test piece of 10 cm in square was disposed on a hot plate which wasset to a temperature of 250° C. The salad oil (soy bean oil) of 1.0 μlwas scattered in approximately fifty points and was dropped on the filmsurfaces to visually check how oil stains changed. In the case of theformer paint only, the salad oil residues remained and became tarry onthe entire surfaces of the test piece.

Then, a system to which 5% calcium silicate by weight was added wasproduced. In this case, the paint was remarkably gelled. However,viscosity was prepared with addition of approximately 10% water. Thefilm had many cracks therein and was inferior in adhesion. The otherphysical properties were good. Stains were removed after five minutesthrough the cleaning test. Then, 10% iron oxide (Fe₂ O₃) was added byweight. The same test was carried out. In this case, the same superiorphysical property of the film as that of the paint itself was obtained.Even at the cleaning test, two or three tar traces each being ofapproximately 2 mmφ remained. But significant capability was obtained.

Then, the same test was carried out about a case where 5% iron oxide byweight and 1% lime aluminate by weight were added.

The film was very superior in physical property (impact resistance orthe like was superior as compared with the paint only). Stainsdisappeared after several minutes. The same tests were repeated seventimes with no tar traces remaining on the surfaces.

Such superior effects as described hereinabove were provided, becauseinstead of multiplication action of both catalysts, some addition of analkaline earth metal partially gelled the paint, with the result thatthe catalyst compound was considered to be adapted to be exposed withoutreacting with the binder, particularly on the surface layer.

In the case of the metallic phosphate group binder, the added amount ofthe catalyst is preferred 5% or more in the oxide of transition metaland 1% or less in alkali rare earth metallic salt or the like.Particularly, a method of jointly using both of them is best. Even inthis system, excessive porosity is not required, considering the demandof the catalytic reaction itself. Approximately 10% porosity will dofrom the viewpoint of the practical film physical properties.

Since a sufficient effect can be exhibited even with porosity, the filmphysical property can be retained at its superior level.

Effects of controlling the tarry condition of oils such as salad oil orthe like have been described. They can be sufficiently effective evenfor the kerosene group or the like, where the tar production is aproblem.

The practical effect in relatively smooth surfaces is larger. In thecase of residues which the catalyst can not remove, e.g., inorganic soilsuch as salt or the like, it is required to wipe the surfaces with acloth or the like. The self-cleaning surfaces are not always complete.In the case of the conventional porous coating, the cloth fibers werecaught therein, being adhered in balls, thus resulting in obstacles topractical application. In this case, problems such as describedhereinabove do not occur.

In terms of the cleaning speed, a fact that the oil is removed throughevaporation cannot be neglected in the cleaning phenomenon. In theconventional porous coating, the oil penetrates into the coating, thuscausing hardening reaction. The oil is not spread on the surfaces,delaying the evaporation. On the other hand, the oil spreads rapidly onthe smooth surfaces and is immediately evaporated. Since approximately70% of salad oil is adapted to be evaporated at a temperature of 250°C., the skilful use of this evaporation contributes towards theadvantageous cleaning speed.

Another advantage is that at a low temperature of 100° C. or less, lesspermeation is provided on the smooth surfaces. In the conventionalporous coating, the oil is evaporated and cracked where the temperatureof the oil permeated portion becomes 250° C. through the followingcooking cycles. Thus, unpleasant smells which are caused through thedecomposition of the oil remain fully within the oven of the cookingdevice. The phenomenon is completely undesirable when the cooking beingperformed is adversely affected with the former unpleasant smells. Thisis one of the disadvantages with the present porous coating. The smoothsurfaces of the present invention free from this problem.

In the relationship between the catalyst addition amount and effect, theeffect can be recognized through addition of approximately 1% by weightin the case of calcium silicate or the like. The calcium silicate islower in specific gravity and is difficult to be distributed into thepaint. But it can be effectively distributed, in small amount, on thecoated wet surfaces and can exhibit its effect as is after the bakingoperation. A highly active catalyst such as calcium silicate, limealuminate or the like is added 30% by weight, using alkali metalsilicate group binder to provide surprisingly superior cleaningcapability. In this case, cracks are likely to be caused in the film,developing into the deterioration of the film physical property. As thelevel of the catalyst addition amount, approximately 4% to 10% by weightis optimum as total amount. However, when the catalyst uses oxides ofTi, Fe, Ni, Co, Cr and Ag, addition of at least 5% or more by weight isrequired, because these metallic oxides are likely to be wrapped in thefilm and furthermore the specific gravity is higher. In this case,addition of approximately 30% by weight hardly damages the film physicalproperty. However, these metallic oxides are somewhat inferior in thecleaning capability of the fatty acid.

Furthermore, dispersion of the catalyst into the paint is required to becarried out using dispersion agent such as ball mill or the like. Thecleaning capability varies depending upon dispersing method and time.When the dispersion is completely performed, the catalyst is completelywrapped in the binder and a catalyst exposed on the surface cannot beprovided. However, when the dispersion is incomplete, the film physicalproperty is deteriorated. Accordingly, the optimum dispersion timeexists. When the catalyst is dispersed by the use of the ordinary ballmill, the catalytic effect is obtained. Optimum dispersion time toensure sufficient film physical property is from half an hour to onehour. This has nothing to do with the type of the binder.

In a film making method for surfaces such as metallic faces, brushpainting, air spraying, electrostatic painting or the like, can beapplied as in ordinary painting. The film is formed from 20μ toapproximately 180μ. The film thickness of approximately 50μ to 150μprovides good physical property. A baking hardening operation isdesirably performed in a hot blast stove. A temperature of approximately350° C. at most for approximately thirty minutes will do. As comparedwith the conventional enamel products, the stove temperature control iswider in tolerance and the baking operation can be performed at lowertemperatures and for shorter period of time, thus resulting in superiorproductivity and extremely advantageous cost. As the foundation face,not only the faces of metals such as iron, aluminum, etc., but also thefaces of ceramics can be coated with paint, so that application can bemade to almost the entire over-inner face of the cooking device and theparts therein. The method of the present invention is considered to besuperior not only in cost, but also in the range of practicalapplication.

As described hereinabove, it has been confirmed that the catalyticcoating of the present invention has novel effects completely differentfrom the range of this type of conventional coating, can exhibit theself-cleaning effects extremely superior in practical application, has aquality level, in coating, equal to or superior to the conventionalenamel product, and has superior characteristics unavailable with theporous coating.

What is claimed is:
 1. A coated metal substrate of aluminum having asurface capable of decomposing oils at about 200° to 300° C. through agasification cracking reaction, said surface being produced by a processconsisting essentially of applying a paint to said metal substrate andsintering said paint, said paint including a catalyst consisting of atleast one or more oxides selected from the group consisting of:(A)oxides of Group 1A or 2A of the Periodic Table, and (B) compoundsrepresented by the formula (MA)_(x) (MB)_(y) (O)_(z) where MA: elementof Group 1A or 2A MB: element of Group 3A or 4A O: oxygen x, y, z:interger said catalyst being the sole effective catalyst for decomposingoils and being present in an amount sufficient to effect suchdecomposition and an inorganic binder, a hardening agent, and a pigment.2. The coated metal substrate as claimed in claim 1, wherein a metallicphosphate is used as said binder.
 3. A method for decomposing oilycooking residues by gasification cracking action which comprisessubjecting the coated metal substrate of claim 1, having oily cookingresidues thereon, to elevated temperatures to effect such decomposition.4. The method according to claim 3 wherein the catalyst in said paint iscalcium aluminate.